There are two stable isotopes of He, 3He (1.37 x 10-4
%) and 4He (>99.99 %). Although ground waters contain 3He
from several sources other than 3H decay (see below), determination
of both 3H and tritogenic 3He (3He*) can
be used as a dating tool (Tolstikhin and Kamensky, 1969; Torgersen et al.,
1977; Takaoka and Mizutani, 1987; Schlosser et al., 1988). While decay
of 3H to 3He commences in the unsaturated zone, dissolved
3He generated from 3H decay can be lost to the atmosphere.
It is not until it enters the ground water that 3He is preserved
because gas transport in the unsaturated zone is rapid relative to transport
in ground water. Once isolated from the atmosphere below the water table,
dissolved 3He concentrations will rise. For this reason, a 3H/3He
age of t = 0 should occur at the water table, and ages should increase
with depth as ground water becomes older. In the case of a fluctuating
water table, t = 0 is set at the seasonal low water table position (Solomon
et al., 1993).
The tritogenic 3He component is calculated by subtracting
from the measured value that due to atmospheric solubility (a function
of recharge temperature), excess air and radiogenic production (by decay
of U/Th-series elements). These different sources of 3He are
identified by measuring concentrations of 4He and other noble
gases (Torgersen et al., 1977, 1979; Weise and Moser, 1987; Schlosser et
3H/3He ages accurately reflect hydraulic ages
(subject to the uncertainties discussed above) in ground-water systems
with piston flow and negligible mixing. However, the concentration ratio
of 3H and 3He at any given point will not always
accurately reflect the ground-water age because the concentration gradients
are usually different and the diffiusion coefficient for 3He
> 3H in water. Underestimates of ages of surface water bodies
have been shown to be the result of diffusive loss of 3He to
the atmosphere (Wuest et al., 1992), and will similarly lead to underestimates
of ground-water ages (Schlosser et al., 1989). Analytical uncertainties
usually result in errors in age estimates of less than 10% (Solomon et
al., 1993). In addition to dating waters, 3He levels have also
been shown to be a function of the vertical-flow velocity (recharge rate)
and dispersivity (Schlosser et al., 1989).
Radiogenic stable 4He has been used to locate areas of deep
ground water discharge. High 4He concentrations are common in
deep ground waters as a result of subsurface a-decay
of natural U- and Th-series elements in rocks and sediments. Mass transfer
between aquifer solids and ground water occurs, resulting in a relationship
between ground-water travel time (or contact time) and the 4He
concentration in ground water. If the 4He solid-to-liquid mass
transfer rate is known, it is possible to use 4He as a quantitative
tracer of ground-water travel times (e.g. Marine, 1979; Andrews and Lee,
1979; Torgersen, 1980; Stue et al., 1992). If the solid-to-liquid mass
transfer rate is governed by the radioactive decay of U and Th, significant
(i.e. measurable) amounts of radiogenic 4He will exist in ground
water after a contact time on the order of 1000 years and can theoretically
continue to accumulate for millions of years. Thus, 4He has
generally been considered to be a hydrologic tracer over time scales of
103 to 106years (e.g. Mazor and Bosch, 1992); however,
a recent study by Solomon et al. (1996) has shown that some aquifer solids
are currently losing ancient 4He (previously trapped within
crystal lattices) at rates that are controlled by solid state diffusion.
If quantified, such high solid-to-liquid mass transfer rates allow ground-water
dating with 4He for water as young as 10 years and possibly
as old as 103 years.
Ground water ages using 4He are calculated by determining
the solid-to- liquid mass transfer rate, and then measuring the 4He
content of ground-water samples. The solid-to-liquid mass transfer rate
can be determined directly in the laboratory or by calibration using ground-water
age data obtained using other (i.e. CFCs or 3H/3He)
dating methods. Once the solid-to-liquid mass transfer rate has been estimated,
ground-water ages are determined by measuring total He and Ne in ground-water
samples, and computing the component of radiogenic 4He. Because
ground water contains 4He from the atmosphere as well as that
released from solid phases, it is necessary to separate radiogenic from
The uncertainty in ground-water ages determined using radiogenic 4He
results mostly from uncertainty in the solid-to-liquid mass transfer rate.
While the age uncertainty using 4He is much greater than with
CFCs, 3H/3He, or 85Kr, the potential for
dating waters in the range of 50 to 1000 (where no other dating methods
are practical) makes the use of radiogenic 4He attractive.
Further information can be found in Clark and Fritz (1997), Environmental
Isotopes in Hydrology, (CRC Press):
Source of text: This review was assembled by Eric Caldwell and Dan
Snyder, primarily drawing from Solomon et al., 1998.
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